1. Field of the Invention
The present invention relates to a cold cathode tube lamp.
2. Description of the Related Art
FIG. 21 is a schematic sectional view of a conventional cold cathode tube lamp. The conventional cold cathode tube lamp shown in FIG. 21 has internal electrodes 2 and 3 inside a glass tube 1. A portion of the internal electrodes 2 and 3 penetrate through the glass tube 1 to protrude outside the glass tube 1, functioning as electrode terminals. In the structure described above, the inside of the glass tube 1 is sealed. A fluorescent substance is applied to the inner wall of the glass tube 1. Into the sealed glass tube 1, in order that the overall pressure inside the glass tube 1 may become 10.7×103 to 5.3×103 Pa (≈80 to 40 Torr), neon and argon are typically sealed with a ratio of 95 to 5, 80 to 20, or the like, and further several milligrams of mercury is enclosed. Note that, instead of mercury, xenon may be enclosed.
When the lamp voltage, i.e., voltage between the internal electrodes, reaches a discharge start voltage VS, discharge starts, whereby mercury and xenon generate ultraviolet rays which causes a fluorescent substance applied to the inner wall of the glass tube 1 to illuminate.
The conventional cold cathode tube lamp shown in FIG. 21 has an equivalent circuit thereof serving as a resistance whose resistance value decreases nonlinearly in accordance with an increase in current, and has a nonlinear negative impedance characteristic like a V-I characteristic shown in FIG. 22 (for example, see JP-A-H7-220888 (FIG. 4)).
As one of the applications of the conventional cold cathode tube lamp shown in FIG. 21, there is a backlight for a liquid crystal display device. When the display screen of the liquid crystal display device is large, a plurality of cold cathode tube lamps is provided in an array. In this case, if a plurality of cold cathode tube lamps can be driven in parallel, only one power supply device can be provided since the same voltage is applied to all the cold cathode tube lamps.
Now, driving a plurality of (for example, three) cold cathode tube lamps in parallel will be discussed. There is a variation in the V-I characteristic among the individual cold cathode tube lamps. The V-I characteristic lines T1 to T3 of the first to third cold cathode tube lamps, respectively, are V-I characteristics shown in FIG. 23. The same alternating voltage is applied to the first to third cold cathode tube lamps, and this alternating voltage is boosted. As a result of boosting, when the alternating voltage reaches a discharge start voltage VS1 of the first cold cathode tube lamp, the first cold cathode tube lamp lights up, and a voltage across the first cold cathode tube lamp decreases due to the nonlinear negative impedance characteristic. The voltage across the second cold cathode tube lamp and the voltage across of the third cold cathode tube lamp agrees with the voltage across the first cold cathode tube lamp; therefore, the aforementioned alternating voltage never reaches a discharge start voltage VS2 of the second cold cathode tube lamp and a discharge start voltage VS3 of the third cold cathode tube lamp. That is, when a plurality of cold cathode tube lamps are simply driven in parallel, only one of the cold cathode tube lamps can be lit up. Therefore, a structure is typically adopted in which a power supply circuit is provided for each cold cathode tube lamp to light up a plurality of cold cathode tube lamps. However, with this structure, the same number of power supply circuits as that of cold cathode tube lamps is required, thus resulting in high costs. This is disadvantageous in terms of reduction in size, weight and cost. Moreover, each cold cathode tube lamp is typically connected to a power supply circuit via a harness (also called a lead wire) and a connector. Thus, this involves much labor in fitting the cold cathode tube lamp, thus resulting in deteriorated assembly efficiency with a lighting device or the like using the cold cathode tube lamp, and also involves much labor in detaching the cold cathode tube lamp. This results in decreased replacement efficiency upon replacement of the cold cathode tube lamp and deteriorated dismantling efficiency upon disposing a lighting device or the like using the cold cathode tube lamp.
As a lamp capable of solving such a problem, an external electrode fluorescent lamp (EEFL) has been developed (for example, see JP-A-2004-31338 and JP-A-2004-39264). FIG. 24 is a schematic sectional view of the external electrode fluorescent lamp. In FIG. 24, portions which are the same as those in FIG. 21 are provided with the same numerals and thus omitted from the detailed description. The external electrode fluorescent lamp shown in FIG. 24 is prepared by removing the internal electrodes 2 and 3 from the conventional cold cathode tube lamp shown in FIG. 21 and forming external electrodes 4 and 5 at end portions of the glass tube 1. In the structure described above, the inside of the glass tube 1 is sealed.
In the external electrode fluorescent lamp shown in FIG. 24, when the lamp voltage, i.e., voltage between the external electrodes, reaches a discharge start voltage VS′, discharge starts, whereby mercury and xenon generate ultraviolet rays which cause a fluorescent substance applied to the inner wall of the glass tube 1 to illuminate.
The inside of the glass tube 1 has a nonlinear negative impedance characteristic, and the external electrodes and the inside of the glass tube 1 are insulated from each other by glass. Thus, the external electrode fluorescent lamp shown in FIG. 24 has an equivalent circuit thereof serving as a serial connected body in which a capacitor is connected to both ends of a resistance whose resistance value decreases nonlinearly in accordance with an increase in current. Therefore, the external electrode fluorescent lamp as a whole has a nonlinear positive impedance characteristic like a V-I characteristic shown in FIG. 25.
Now, driving a plurality of (for example, three) external electrode fluorescent lamps in parallel will be discussed. There is a variation in the V-I characteristic among the individual external electrode fluorescent lamps. The V-I characteristic lines T1′ to T3′ of the first to third external electrode fluorescent lamps, respectively, are V-I characteristics shown in FIG. 26. The same alternating voltage is applied to the first to third external electrode fluorescent lamps, and this alternating voltage is boosted. As a result of boosting, when the alternating voltage reaches a discharge start voltage VS1′ of the first external electrode fluorescent lamp, the first external electrode fluorescent lamp lights up. Then, the alternating voltage described above increases with an increase in the output from the power supply device. Then, when the alternating voltage reaches a discharge start voltage VS2′ of the second external electrode fluorescent lamp, the second external electrode fluorescent lamp lights up, and when the alternating voltage reaches a discharge start voltage VS3′ of the third external electrode fluorescent lamp, the third external electrode fluorescent lamp lights up. That is, even when a plurality of external electrode fluorescent lamps are simply driven in parallel, all the plurality of external electrode fluorescent lamps can be lit up.
Due to the arrangement of the external electrodes on the outer circumference of the glass tube, in a lighting device or the like using an external electrode fluorescent lamp, a holding jig formed of a resilient metal member (for example, spring steel) clips the external electrode of the external electrode fluorescent lamp under the influence of its resilient characteristic, so that a power can be supplied to the external electrode fluorescent lamp via the holding jig. Such a method provides an advantage that the external electrode fluorescent lamp can be fitted and detached easily.
However, in the external electrode fluorescent lamp, the glass lying between the external electrode and the inner space of the glass tube corresponds to a dielectric body that is clipped by an electrode of a capacitor as one component of an equivalent circuit of the external electrode fluorescent lamp. Thus, charged particles hit against the inner wall of the glass tube opposing the external electrode, so that the inner wall of the glass tube is locally subjected to spattering. Then, once the inner wall of the glass tube is subjected to spattering, the electrostatic capacitance of the portion subjected to this spattering increases. Thus, the charged particles intensively hit the portion subjected to this spattering and a pin hole finally opens, and then the sealing condition inside the glass tube can no longer be maintained. Thus, the external electrode fluorescent lamp has been suffering from a problem with reliability.